That’s a good question. I have experimented with small images (500 pixels long dimension) to make sure there would be enough room for the caption. I don’t want the photo to be too small though because it’s a book about photography, and if the photos are 300 pixels or smaller on the long edge it will be hard to see detail.
Is there a size that tends to work best when using “Keep With Next”?
That’s the fundamental challenge for ebook formatting: the range of possible reader screen sizes is enormous, from phones all the way to large desktop displays, and most reader software allows the user to change font size (and therefore page size) at will.
My advice would be to use an image size large enough to show the detail you want on “typical” (however you choose to define that) reader devices, and let the captions fall where they may. Most ebook users are familiar with the issue and won’t be surprised if the captions roll over to the next page.
There are no directly comparable features to such a thing as “keep with next” on a Kindle or other HTML-based format to my knowledge, and nothing will be exported by Scrivener in replacement of these codes. There probably are some CSS tricks that could be employed (outside of the realm of Scrivener as post-compile design work).
By the way, from your description it sounds like you are inserting KWN codes for both the caption and the image. In most cases you would want one for the image line, so that it keeps the caption (what is next) with it, but you probably don’t want the caption and image glued as well to the paragraph following the caption, too.